Separate alpha from color in Shader Editor
It's only a partial answer to the body of the question and it's a bit hacky. You can use a ColorRamp node, set the Factor to 0 and tweak the color and alpha values of the first stop. This would effectively emulate a RGB node with an alpha output.
But you you can't use it to combine RGB values and an Apha value since the color ramp node doesn't take inputs for its color stops.